The present invention relates in general to steam generators and, in particular, to a new and useful once-through steam generator having a high-temperature heat accumulator or starter connected upstream of an economizer or evaporator of the steam generator.
Once-through steam generators are used, for example, in combined cycle power stations as waste heat recovery equipment of gas turbines. Such steam generators, which are constructed as once-through boilers, make use of hot exhaust gases of the gas turbines for the purpose of generating additional steam. These exhaust gases generally have a temperature of over 500.degree. C. They can be operated for short periods of time in order to cover peak current requirements.
Known steam generators of this type, for example, as disclosed in ETV-Register 45/86, page 59, or Power, April 1985, page 118, consist substantially of an economizer, an evaporator, which is connected downstream, and a superheater connected to the evaporator, which are arranged one above the other in a boiler pass. During operation, the exhaust gas mass flow, which is still hot, is conducted through the boiler pass, first through the superheater, then through the evaporator, and lastly through the economizer. The feed water entering the economizer is preheated therein, then arrives at the evaporator where it is evaporated to form wet steam. In the superheater, the wet steam is finally converted to superheated steam and is then fed to a steam turbine. An outlet header is generally included between the evaporator and superheater and between the superheater and the steam turbine.
When the generated steam is not required, e.g., during times of low current consumption, the steam generator is shut down. In systems having an exhaust gas diverter, after interrupting the feed-water feed, the exhaust gases are by-passed from the still running gas turbine through the exhaust gas diverter duct to a chimney. The boiler then cools off.
If no exhaust gas diverter is available, which may be the case in once-through boilers, the water content still present in the boiler when shut down is evaporated by the exhaust gases of the gas turbine which continue to flow through the boiler. The economizer, evaporator and superheater then become empty and their tubes become heated to exhaust gas temperature. In order to put this steam generator which is now in a heated state into operation again, the economizer, the evaporator and the superheater must first be cooled-off to their normal operating temperature by means of the feed water, and the water supply of the boiler must be restored.
When starting up the cooled off steam generator with an exhaust gas diverter, a diverter damper remains open so that the hot exhaust gas mass flow is guided into the atmosphere in a circuitous manner around the economizer, evaporator and superheater so as to be completely unused. During this start-up phase, the economizer and evaporator are refilled with feed water, after which the gas diverter is closed and the hot exhaust gas mass flow is again guided through the boiler pass. To a great extent, this prevents temperature shock, such as would occur by the entry of cold feed water into the boiler which is heated to the exhaust gas temperature.
The use of the gas diverter is costly and maintenance-intensive. The diverter dampers for opening and closing the diverter duct which have correspondingly large dimensions and which block and open the path of the exhaust gas through the boiler, involve considerable problems with respect to tightness. Because of the different local temperatures and the enormous size of the dampers, the dampers often warp and then no longer close so as to be tight. Also, the control and blocking devices for the diverter dampers are costly. Since the diverter duct must conduct the entire volume of exhaust gas around the boiler, it requires additional space for the diverter ducts and increases the costs of the heat recovery system.
If no exhaust gas diverter is provided, the empty steam generator is heated at the exhaust gas temperature after the "evaporization" of all water from the boiler tubes. A disadvantage in these steam generators is that, when starting up the hot steam generator, that is when refilling the boiler with feed water, there are great differences in temperature between the feed water, the economizer and the evaporator tubes which produce large thermal stresses.